It is known to use buffer layers in organic electronics, such as organic light emitting diodes (OLED) or organic photovoltaic cells (OPV), in order to increase device efficiency and life-time. Such buffer layers comprise metal oxides, such as ZnO, TiOx, WOx, NiO, NbyOx, or doped metal oxides, such as Al-doped ZnO (“AZO”). Generally, such metal oxides in particulate form are known. Typically, the above named oxidic buffer layers are manufactured by thermal evaporation under high vacuum; which is disadvantageous in terms of low-cost, large-area manufacturing processing.
It is also known that polymer solar cells (OPV) offer a promising approach for a low-cost and flexible photovoltaic technology with certified efficiencies exceeding 10%. Before widespread commercialization, large area production and stability issues have to be solved. For the reliable large area production with high yield and low shunts, thick, stable, robust and printable buffer layers are a prerequisite.
It is also known to use channel layers in transistors, particularly in TFTs. Such channel layers comprise metal oxides, such as ZnO, or mixed oxides, such as indium zinc oxide (ZITO), indium gallium zinc oxide (IGZO) or ZnSnO3. Generally, such metal oxides in particulate form are known. As discussed above, such oxidic layers are manufactured by thermal evaporation under high vacuum; which is disadvantageous in terms of low-cost, large-area manufacturing processing.
Leidolph et al. (EP2157053) describe specific ZnO particles, which are optionally coated, and the manufacturing thereof. It is further speculated about the use of such particles, for example in solar cells.
Rohe et al. (WO2006/092443) describe surface-modified ZnO particles and the manufacturing thereof. It is further speculated about the use of such particles in photogalvanic and photoelectric cells.
Yip et al. (Adv. Mater., 2008, 20, 2376-2382) report on a solution-processed nanoparticulate ZnO buffer layer in organic solar cells. The coating liquid is a suspension of unmodified ZnO nanoparticles in 1-butanol. This suspension was applied on organic layers without damaging them and secondly, a temperature post-treatment of <100° C. was sufficient. Direct contact of the deposited ZnO layer with a silver electrode resulted in low performance devices with low fill factors. In order to improve the contact between the ZnO and the silver, Yip et al apply a self assembled monolayer (SAM) at the interface ZnO/Ag. Application of a SAM layer includes a separate and additional processing step, which is considered disadvantageous.
Stubhan et al. (Solar Energy Materials & Solar Cells, 107 (2012), 248-251) report on solution-processed AZO ETL layers which are produced by a sol-gel technique. A temperature treatment of such layers below 150° C. is sufficient in order to obtain high performance organic solar cells. But still, this material is limited to inverted device architectures because deposition of the sol-gel precursor liquid on top of an active organic layer will damage this layer. This disadvantageous effect is shown in the present application (see examples for the sol-gel produced AZO (LT-AZO). Stubhan et al (Adv. Energy Mater. 2012, 532-535) further disclose methods for increasing the fill factor of inverted solar cells using specific phosphonic anchored SAMs. As apparent from that document, FIG. 1, the AZO nanoparticles are not coated. Rather, a further SAM-layer comprising specific phosphates on top of the AZO nanoparticles applied. Although these layers also result in good PCE values, manufacturing of the devices is difficult, due to the extra coating to obtain a SAM layer.
Brabec (US2007/0289626) discusses photovoltaic cells comprising electrodes with conductive particles. However, this document does not provide any specific teachings of such particles, it broadly suggests its use as component of a photovoltaic cell's electrode.
Puetz et al. (Solar Energy Materials, 2011, 579) disclose unmodified, Indium doped Zinc oxide nanoparticle suspension and its use between an active layer and a silver electrode.
Until now, no metal oxide buffer layers (particularly no ZnO or AZO ETL layers) for organic electronics are known which are present between an active organic layer and a silver electrode (“inverted architecture”) meeting industrial demands. The reason for this is that either the coating liquid is damaging the active layer (see above, Stubhan et al) or is forming an insufficient contact with the silver electrode (see above, Yip et al).